The Riddle of your Heart
by bumblebeecas
Summary: "You can solve the riddle of your heart." *** Tony and Bucky build a time machine and it leads to disastrous consequences. *** Part of the "Infinity War and Endgame never happened" AU
1. Chapter 1

Tony Stark was anything but a patient man, that everyone in the Avengers Headquarters knew.

"Look, can't you just leave it till morning-"

"Bruce, I've almost got it, just chill," Tony replied as he furiously typed and yelled commands at JARVIS.

"I'm just saying Tony, maybe we could-"

"Bruce, if you're gonna keep talking, go up stairs and play Pictionary with Cap."

Bruce Banner didn't reply because reasoning with Tony always ended up the same way: Bruce losing the fight. He wasn't really in the mood to mess around with Tony's stupid experiment anyways, so he sighed deeply as he tended to do when he worried about his friends and left the lab, planning on asking Steve Rogers to have a conversation with the engineer.

Tony didn't notice Bruce leave until JARVIS said something. It wasn't like he was mad at the guy, it was just that he worked better alone and for long hours. He wasn't one for sleep, that everyone in the compound knew. The only other person who seemed to never sleep was James Barnes, and Tony actually didn't really know what he did when he wasn't around Tony. Right now he was at least sitting in the lab quietly, watching. He liked to watch Tony work and the younger man didn't actually know why. Steve said something about Howard and Bucky being close during the war, but Tony never dwelled on any thought of his father long enough to find reasoning in it, so he let the hundred-year-old stay, occasionally grabbing a screwdriver or part for him.

"You're quiet," Tony commented, not really happy with the lack of talking now that Bruce had left (even though he had kicked him out for talking).

"Aren't I always?" Bucky replied softly.

Tony shrugged. "Yeah, I suppose. I always think you're plotting my death or something, that's all."

Bucky shrugged. "Who knows."

Tony liked to think that was a joke. "Will you grab-"

"Yeah, the Philips-head, sure," Bucky slipped off the counter of the kitchen area and moved towards the tool box, patting Dum-e as he walked past.

"Are you a mind reader now too?" Tony asked as he took the screwdriver from Bucky's hand, the flesh one.

"No, I was watching and you were finishing up that motor-"

"Do you even know what I'm doing?"

"Don't patronize me," Bucky replied as he slipped back onto his perch.

Tony shook his head. He preferred it when the assassin didn't talk, anyways.

"Is it what I think it is?" Bucky asked.

"Wow, this must be a new record of how many words you've said in a single period. Alert the media."

"Answer the question, fat-head."

"Man, I love your 40s slang, I really do."

Bucky folded his arms across his chest, glaring. Tony sighed, throwing the screwdriver down. "Well, that depends on what you think it is, doesn't it, Barnes?"

"It's a time machine."

Tony whistled. "Never let anyone tell you that you're just a pretty face."

"Why are you building a time machine? Are you going to _Quantum Leap?_"

"Man, I told you to stop watching that shit."

Bucky had gotten up again and was now right next to Tony, who had sat up from where he crouched next to the massive machine. Okay, yes, it _was_ a time machine. It was more of a _Let's See If I Can Really Do It_ project, but maybe there were things Tony wanted to fix in his life. Maybe.

"What's it to you, Barnes? You go back and mess with your future, you won't be here to tell the tale. There's _rules_."

"I know there's _rules_! I've seen all five seasons of Quan-"

"Do _not_ start with that again!" Tony yelled, putting his hand up to silence the other man.

The two were silent.

Finally, after what seemed like ages, Tony spoke. "What would you use it for?"

Bucky smiled softly. "I'd seem my family again. You?"

"Same."

Families were a tough topic for all the Avengers, but especially Tony since the guy who'd killed them was now his sort of lab assistant. Though Tony wasn't patient, he was understanding. He still remembered reading through Barnes' file and watching the videos of HYDRA torturing the young man. It hurt Tony and reminded him of when he was held hostage by terrorists. At least the two could bond now.

"What if we did it?" Tony finally asked, looking over at Bucky hopefully.

"Like, now?"

"No, I have a few things still left to do, but Barnes, what if we saw them again? Wouldn't you like that?"

Bucky thought of his younger sister. He would love to see her again.

"Isn't it dangerous?" Bucky asked, looking hesitant to even ask.

"I'll hold your hand," Tony joked.

Bucky laughed, though it was soft and more of a chuckle. "Okay, well, what do you need?"

"Well, for you to watch _Back to the Future_."


	2. Chapter 2

"Well?" Tony asked, hands on his hips and goggles still perched on top of his head.

"Hmm," Bucky hummed, still staring at the television as the credits rolled.

"Say something," the engineer said, staring at the bionic man impatiently.

"Marty McFly is no Sam Beckett but-"

"Oh, for crying out loud, why do I even _try_ with you?!"

Tony sighed heavily, crossing over to the work space once again. It had been exactly five hours and forty-two minutes since Tony had placed Bucky in front of the flat screen TV he had in the workshop (for relaxation purposes) and made him watch all three _Back to the Future_ movies. It wasn't the most productive use of anyone's time, but Tony couldn't help but want him distracted as he worked on this time machine, which still sounded unbelievably _childish_ if he thought too long and hard about it.

"Well, what did you learn?" Tony asked from his computers, looking expectantly over at the older man.

He didn't reply for a minute.

"Don't have premarital sex with the younger version of my mother," Bucky offered.

Tony genuinely face palmed.

"I mean about _time travel_, you dipstick!"

"Well, doesn't that count as 'about time travel?' Isn't that an important lesson we should all remember?"

Bucky was smiling slightly and that was the only indication Tony had that told him he was joking.

"You know, you talk so dead-pan all the time, I thought you were being serious!" Tony shook his head as his rolled his chair over to the machine, inspecting it once again.

"Maybe I was being serious. _Deadly serious_," Bucky replied, his voice flat and threatening.

"Stop that and come here."

Bucky rose from the couch, walking over to where Tony sat.

"Is that the time machine?" Bucky asked, his voice hushed as if there were others around that couldn't know about the marvelous machine Tony Stark had just built.

"No, it's the downstairs vacuum, just though it could use some LED lights installed- _yes, it's the time machine!_"

"No need to be so snappy," Bucky rolled his eyes, his left hand running gently against the exterior.

The machine was not a Delorean, nor a blue police box or quantum accelerator or hot tub. It was a dull grey in color with slim clear tubes positioned on top of the box-like structure. The tubes would fill with antimatter to power the fusion engines. The machine, contrary to pop-culture belief, was not large. It was roughly the size of a shoe box, if you were to buy a specific pair of shoes that had a box size of 14x18. Inches, of course, because we're uncivilized. There was a holographic screen coming from the side of the box, allowing one to command the machine and say where they would like to go, or, _when_ they would like to go.

"That's it?" Bucky finally asked, breaking the revolutionary silence that had once encompassed the entirety of the workshop.

"What do you mean _'is that it'_?! That's a damn time machine, a _working_ time machine! What, could _you_ do better?"

Bucky looked over, his face unable to be read. "No, I just mean... isn't it a bit... _small_?"

Tony groaned as he ran his hands over his face. "Yes, it's small. It's not the whole thing, dumb ass."

"Well, then where's the whole thing?" Bucky asked, his arms crossed across his chest in an impatient manner.

Tony pointed with his thumb in a jerky motion towards a tall and thin oval-shaped machine that was hollow in the middle as to allow users to walk through it.

"That would be the part we actually travel though, Barnes."

Bucky walked over peaking his head through the opening as if expecting something to happen instantly.

"Okay. Turn it on," Bucky finally said, through with his examinations.

"What?"

"Turn it on. Let's take it for a test drive."

"That's not how testing time machines work."

Bucky frowned. "That is exactly how testing time machines work, Tony."

"Well, don't they usually test on dogs or apples first?"

Bucky smiled a genuine smile, looking over slyly. "We could do that. Or, we could trust your skills and knowledge and test it ourselves."

Tony was torn. On one hand, Bucky was playing him like a violin. But, on the other, he really didn't think any test was necessary. He was a genius, did he really need to test the thing?

"Okay. Fine. I get first pick, though."

Bucky shrugged. "Seems fair."

Tony crossed over to the small box, typing rapidly into the machine until it stirred with motion, the tubes lighting up and hence, the oval portal illuminating as well.

"November 12, 1986, MIT," Tony commanded.

"Right away, Sir," JARVIS spoke.

"What happened that day?" Bucky asked as the oval portal began to shine a blue-ish color.

"Well, I suppose you'll find out," Tony said.

The two men walked forth into the portal, disappearing from 2019 entirely.


End file.
